Refrigeration systems used in the food cold storage industry sometimes incorporate a desiccant mechanism known as a Munters wheel to dry the outside air that is pulled into the system upstream of the evaporator, so as to reduce frost formation. The wheel is a honeycomb structure coated with a desiccant material, such as a zeolite, that will absorb moisture from the outside air blown over it. One half of the wheel turns slowly through the outside air intake duct, continually picking up moisture. As it does so, it releases the latent heat of the moisture to sensible heat, warming the dried air as it passes through the wheel. Therefore, the evaporator is forced to cool air that has simply exchanged latent for sensible heat, with no net reduction in its effective cooling load. The other half of the wheel turns through another, separate duct adjacent to the intake duct, through which independently heated air is blown. The heated air passes through the other half of the wheel and is exhausted to the outside. This continually removes moisture from the other half of the desiccant wheel to regenerate it before it rotates back into the intake duct. The extra energy cost for desiccant regeneration is considered worthwhile because of the need for dried refrigerated air in such applications. It is also known, in the residential environment, to provide a metal "thermal" wheel, separate from and downstream from the desiccant wheel, to cool and remove some of the latent heat released by the desiccant. Such a system also relies entirely on water, to the exclusion of refrigerant, and is therefore not usable in a mobile application.
In automotive air conditioning systems, condensation of water vapor on the evaporator surface is typically relied on to dehumidify the air, but with all the attendant condensation problems of microbe growth and corrosion acceleration. However, some vehicle dehumidification systems have been proposed in which vehicle cabin or interior air is forcibly blown over a bed of desiccant and recirculated to the cabin. These systems operate independently of, and also subsequent to, the operation of the basic air conditioning (cooling) system, and represent a good deal of added cost and space. A practical, compact and energy efficient mechanism for dehumidification that worked in conjunction with the air conditioning system could be a decided advantage. It would, of course, provide drier cabin air, which is more comfortable. Furthermore, if the outside intake air could be dried upstream of the evaporator, it would substantially reduce evaporator condensation.